先来看一段简单的代码，稍微有点并发知识的都可以知道打印出结果必然是一个小于20000的值
package com.example.test.cas;
import java.io.IOException;
/**
* @author hehang on 2019-10-09
* @description
*/
public class LockDemo {
private volatile int i;
public void add(){
i++;
}
public static void main(String[] args) throws IOException {
LockDemo lockDemo = new LockDemo();
for (int i = 0; i <2 ; i++) {
new Thread(() ->{
for (int j = 0; j <10000 ; j++) {
lockDemo.add();
}
}).start();
}
System.in.read();
System.out.println(lockDemo.i);
}
}
改进一下，使用jdk给我们提供的原子操作类，达到了我们预想的结果
package com.example.test.cas;
import java.util.concurrent.atomic.AtomicInteger;
/**
* @author hehang on 2019-10-14
* @description
*/
public class AtomicTest {
public static void main(String[] args) throws InterruptedException {
// 自增
AtomicInteger atomicInteger = new AtomicInteger(0);
for (int i = 0; i < 2; i++) {
new Thread(() -> {
for (int j = 0; j < 10000; j++) {
atomicInteger.incrementAndGet();
}
}).start();
}
Thread.sleep(2000L);
System.out.println(atomicInteger.get());
}
}
下面就来探究下jdk为我们提供的原子操作类的原理，基于java native方法实现一个自己原子操作类
package com.example.test.cas;
import sun.misc.Unsafe;
import java.io.IOException;
import java.lang.reflect.Field;
/**
* @author hehang on 2019-10-09
* @description
*/
public class LockCASDemo {
private volatile int i;
private static Unsafe unsafe;
private static long offset;
static{
try {
Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafe.setAccessible(true);
unsafe = (Unsafe) theUnsafe.get(null);
offset = unsafe.objectFieldOffset(LockCASDemo.class.getDeclaredField("i"));
} catch (Exception e) {
e.printStackTrace();
}
}
public void add(){
int curent;
int value;
do{
curent = unsafe.getIntVolatile(this,offset);
value = curent+1;
}while (!unsafe.compareAndSwapInt(this,offset,curent,value));
}
public static void main(String[] args) throws IOException {
LockCASDemo lockDemo = new LockCASDemo();
for (int i = 0; i <2 ; i++) {
new Thread(() ->{
for (int j = 0; j <10000 ; j++) {
lockDemo.add();
}
}).start();
}
System.in.read();
System.out.println(lockDemo.i);
}
}
实现这样一个类的要点有：1、基于反射机制获取UnSafe对象2、利用UnSafe对象获取属性偏移量，然后调用compareAndSwapInt方法，比较和替换是硬件同步原语，处理器提供了基于内存操作的原子性保证。
以上的代码未免麻烦，因此jdk为我们封装了一些原子操作类来简化使用，打开这些原子操作类的源代码，可以发现其内部实现就是循环+调用native方法(比较替换)，常用的原子操作类如下：
cas存在的三个问题
1、循环+cas，自旋的实现让cpu处于高频运行状态，争抢cpu执行时间，如果并发太高，部分线程长时间执行不成功，带来很大的cpu消耗
2、只能针对单个变量实现原子操作
3、出现ABA问题
针对第一个问题，jdk1.8为我们提供了增强版的计数器，内部利用分而治之的思想来减少线程间的cpu争抢，提高并发效率，具体可以看下面的测试
package com.example.test.cas;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.LongAdder;
/**
* @author hehang on 2019-10-14
* @description asd
*/
public class LongAdderDemo {
private long count = 0;
// 同步代码块的方式
public void testSync() throws InterruptedException {
for (int i = 0; i < 3; i++) {
new Thread(() -> {
long starttime = System.currentTimeMillis();
while (System.currentTimeMillis() - starttime < 2000) { // 运行两秒
synchronized (this) {
++count;
}
}
long endtime = System.currentTimeMillis();
System.out.println("SyncThread spend:" + (endtime - starttime) + "ms" + " v" + count);
}).start();
}
}
// Atomic方式
private AtomicLong acount = new AtomicLong(0L);
public void testAtomic() throws InterruptedException {
for (int i = 0; i < 3; i++) {
new Thread(() -> {
long starttime = System.currentTimeMillis();
while (System.currentTimeMillis() - starttime < 2000) { // 运行两秒
acount.incrementAndGet(); // acount++;
}
long endtime = System.currentTimeMillis();
System.out.println("AtomicThread spend:" + (endtime - starttime) + "ms" + " v-" + acount.incrementAndGet());
}).start();
}
}
// LongAdder 方式
private LongAdder lacount = new LongAdder();
public void testLongAdder() throws InterruptedException {
for (int i = 0; i < 3; i++) {
new Thread(() -> {
long starttime = System.currentTimeMillis();
while (System.currentTimeMillis() - starttime < 2000) { // 运行两秒
lacount.increment();
}
long endtime = System.currentTimeMillis();
System.out.println("LongAdderThread spend:" + (endtime - starttime) + "ms" + " v-" + lacount.sum());
}).start();
}
}
public static void main(String[] args) throws InterruptedException {
LongAdderDemo demo = new LongAdderDemo();
demo.testSync();
demo.testAtomic();
demo.testLongAdder();
}
}
三种方式在同样的时间内，自增的数值如下，可以看到LongAdder的效率更高一些
SyncThread spend:2000ms v23074332 SyncThread spend:2000ms v23094924 AtomicThread spend:2000ms v-38137398 AtomicThread spend:2000ms v-38152694 SyncThread spend:2011ms v23094924 AtomicThread spend:2000ms v-38416095 LongAdderThread spend:2000ms v-40097562 LongAdderThread spend:2000ms v-40606405 LongAdderThread spend:2001ms v-40917467 Process finished with exit code 0
针对第二个问题，我们只能通过加锁或者其它手段其处理，这里不做展开
针对第三个问题，我们先模拟出以下的场景来展示这个问题
package com.example.test.cas.aba;
/**
* @author hehang on 2019-10-14
* @description
*/
public class Node {
public final String item;
public Node next;
public Node(String item) {
this.item = item;
}
@Override
public String toString() {
return "item内容:" + this.item;
}
}
package com.example.test.cas.aba;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
/**
* @author hehang on 2019-10-14
* @description
*/
// 实现一个 栈（后进先出）
public class Stack {
// top cas无锁修改
AtomicReference<Node> top = new AtomicReference<Node>();
public void push(Node node) { // 入栈
Node oldTop;
do {
oldTop = top.get();
node.next = oldTop;
}
while (!top.compareAndSet(oldTop, node)); // CAS 替换栈顶
}
// 为了演示ABA效果， 增加一个CAS操作的延时
public Node pop(int time) throws InterruptedException { // 出栈 -- 取出栈顶
Node newTop;
Node oldTop;
do {
oldTop = top.get();
if (oldTop == null) {
return null;
}
newTop = oldTop.next;
if (time != 0) {
System.out.println(Thread.currentThread() + " 休眠前拿到的数据" + oldTop.item);
TimeUnit.SECONDS.sleep(time); // 休眠指定的时间
}
}
while (!top.compareAndSet(oldTop, newTop));
return oldTop;
}
}
package com.example.test.cas.aba;
/**
* @author hehang on 2019-10-14
* @description
*/
public class Test {
public static void main(String[] args) throws InterruptedException {
Stack stack = new Stack();
stack.push(new Node("B"));
stack.push(new Node("A"));
Thread thread1 = new Thread(() -> {
try {
System.out.println(Thread.currentThread() + " 拿到数据：" + stack.pop(3));
// 再继续拿，就会有问题了，理想情况stack出数据应该是 A->C->D->B，实际上ABA问题导致A-B->null
System.out.println(Thread.currentThread() + " 拿到数据：" + stack.pop(0));
System.out.println(Thread.currentThread() + " 拿到数据：" + stack.pop(0));
System.out.println(Thread.currentThread() + " 拿到数据：" + stack.pop(0));
} catch (Exception e) {
e.printStackTrace();
}
});
thread1.start();
Thread.sleep(300); // 让线程1先启动
Thread thread2 = new Thread(() -> {
Node A = null;
try {
A = stack.pop(0);
System.out.println(Thread.currentThread() + " 拿到数据：" + A);
stack.push(new Node("D"));
stack.push(new Node("C"));
stack.push(A);
} catch (Exception e) {
e.printStackTrace();
}
});
thread2.start();
}
}
在上面的例子中我们想实现一个栈，单线程情况下是没有任何问题的，但是在并发场景下就会出现丢数据的问题，运行结果：
Thread[Thread-0,5,main] 睡一下，预期拿到的数据A Thread[Thread-1,5,main] 拿到数据：item内容:A Thread[Thread-0,5,main] 拿到数据：item内容:A Thread[Thread-0,5,main] 拿到数据：item内容:B Thread[Thread-0,5,main] 拿到数据：null Thread[Thread-0,5,main] 拿到数据：null Process finished with exit code 0
好在jdk为我们考虑了这个问题，提供了AtomicStampedReference和AtomicMarkableReference，前者基于时间戳，后者基于标记位来对同样的数据做区分，从未避免了ABA问题
package com.example.test.cas.aba;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicStampedReference;
/**
* @author hehang on 2019-10-14
* @description
*/
public class ConcurrentStack {
AtomicStampedReference<Node> top = new AtomicStampedReference<Node>(null,0);
public void push(Node node){
Node oldTop;
int v;
do{
v=top.getStamp();
oldTop = top.getReference();
node.next = oldTop;
}
while(!top.compareAndSet(oldTop, node,v,v+1));
// }while(!top.compareAndSet(oldTop, node,top.getStamp(),top.getStamp()+1));
}
public Node pop(int time){
Node newTop;
Node oldTop;
int v;
do{
v=top.getStamp();
oldTop = top.getReference();
if(oldTop == null){
return null;
}
newTop = oldTop.next;
try {
if (time != 0) {
System.out.println(Thread.currentThread() + " 睡一下，预期拿到的数据" + oldTop.item);
TimeUnit.SECONDS.sleep(time); // 休眠指定的时间
}
}
catch (InterruptedException e) {
e.printStackTrace();
}
}
while(!top.compareAndSet(oldTop, newTop,v,v+1));
// }while(!top.compareAndSet(oldTop, newTop,top.getStamp(),top.getStamp()));
return oldTop;
}
public void get(){
Node node = top.getReference();
while(node!=null){
System.out.println(node.item);
node = node.next;
}
}
}
package com.example.test.cas.aba;
/**
* @author hehang on 2019-10-14
* @description
*/
public class Test2 {
public static void main(String[] args) throws InterruptedException {
ConcurrentStack stack = new ConcurrentStack();
stack.push(new Node("B"));
stack.push(new Node("A"));
Thread thread1 = new Thread(() -> {
try {
System.out.println(Thread.currentThread() + " 拿到数据：" + stack.pop(3));
// #再继续拿，就会有问题了，理想情况stack出数据应该是 A->C->D->B，实际上ABA问题导致A-B->null
System.out.println(Thread.currentThread() + " 拿到数据：" + stack.pop(0));
System.out.println(Thread.currentThread() + " 拿到数据：" + stack.pop(0));
System.out.println(Thread.currentThread() + " 拿到数据：" + stack.pop(0));
} catch (Exception e) {
e.printStackTrace();
}
});
thread1.start();
Thread.sleep(300); // 让线程1先启动
Thread thread2 = new Thread(() -> {
Node A = null;
try {
A = stack.pop(0);
System.out.println(Thread.currentThread() + " 拿到数据：" + A);
stack.push(new Node("D"));
stack.push(new Node("C"));
stack.push(A);
} catch (Exception e) {
e.printStackTrace();
}
});
thread2.start();
}
}
结果如下：
Thread[Thread-0,5,main] 睡一下，预期拿到的数据A Thread[Thread-1,5,main] 拿到数据：item内容:A Thread[Thread-0,5,main] 睡一下，预期拿到的数据A Thread[Thread-0,5,main] 拿到数据：item内容:A Thread[Thread-0,5,main] 拿到数据：item内容:C Thread[Thread-0,5,main] 拿到数据：item内容:D Thread[Thread-0,5,main] 拿到数据：item内容:B Process finished with exit code 0
原文地址：https://www.cnblogs.com/hhhshct/p/11676923.html
时间： 2024-07-31 20:21:36